Electroceramic component

ABSTRACT

An electric component includes a first base body that connects to a first contact, and an ally conducting element positioned along a current path between the first contact and a second contact. The electrically conducting element melts when an operating voltage of the electric component is exceeded. The electric component also includes an electrically insulating material that substantially surrounds the electrically conducting element and that is arranged so as to prevent an electric flashover between regions of the current path that are bridged by the electrically conducting element.

TECHNICAL FIELD

This patent application describes an electroceramic component with atemperature fuse.

BACKGROUND

Electroceramic components with a temperature fuse includes varistors,for example The base body of such components is often produced from amixture of various metal oxides, such as zinc oxide. Varistors have anon-linear voltage-dependent resistance change that is used to protectan electric circuit from overvoltage. The resistance value of a varistordrops as the applied voltage increases.

When an overvoltage appears (e.g., operating voltage that exceeds apermitted limit value for the varistor), current passing through thevaristor rises sharply. As a result, the varistor heats-up. Along-lasting overvoltage can lead to overheating and set-off a fire.

A varistor fuse element for protecting an electric circuit fromovervoltage and overheating is known from printed document DE 331 85 88.It consists of a mechanical construction in which solder with a lowmelting point is applied to the varistor, which establishes a springcurrent conductor. When an overvoltage and associated overheatingappears, the solder melts, whereupon the spring structure immediatelyproduces an irreversible low-resistance short circuit with a secondcurrent conductor. In this case, the spring structure also increases thedistance between the spring current conductor and the varistor, in orderto prevent an electric flashover. A disadvantage of this mechanicalstructure is in the fact that it can be achieved only at great cost.

In printed document JP 04 151 804 A, a temperature fuse is disclosed,that is integrated into a varistor housing, and that is connectedthrough an electric line to an internal electrode of the varistor. Thetemperature fuse, in this case, is surrounded by a material that permitsheat to be conducted between the varistor and the temperature fuse. Incase of overheating of the varistor due to long-lasting overvoltage, theheat of the varistor can thereby be transferred to the temperature fuseand trigger it. The disadvantage of this structure is that thetemperature fuse is not in direct thermal contact with the varistor.Therefore, because of losses during heat transfer, the fuse is triggeredonly at higher temperatures.

SUMMARY

This patent application describes an electroceramic component with atleast a first ceramic base body. In normal operation, in which theoperating voltage does not exceed a predetermined limit value, a currentflow occurs between two electrical contacts through the first ceramicbase body and an electrically conducting piece. The first ceramic basebody contacts the electrically conducting piece and is therefore indirect heat contact with it. When the operating voltage is exceeded, thefirst ceramic base body heat up strongly due to the increasing loss, sothat the electrically conducting body is also heated up. Theelectroceramic component is implemented in such a way that theelectrically conducting piece melts beyond a certain temperature, andcurrent flow is thereby interrupted. An electrically insulating materialprevents an electric flashover between the electrically conductingregions contacted by the electrically conducting piece and thus assuresa reliable interruption of the current path.

Advantages over the state of the art include that the electricallyconducting piece is in direct heat contact with the first ceramic basebody. Because of this, no heat transfers associated with losses willoccur through enclosing the first surrounding body. The overheating fusecan therefore be triggered at low temperatures at the first base body,and it is therefore significantly more sensitive than traditional fuses.Another advantage is that, after the electrically conducting piecemelts, an electric flashover can be prevented in a simple way by theelectrically insulating material. No expensive mechanical springstructures are necessary to move the contacts away from each other afterthe electrically conducting piece melts.

The first base body can contain, for example, a varistor ceramic basedon zinc oxide. The electrically conducting piece is advantageously asolder with a low melting point, for example, a melting point betweenabout 80° C. and 180° C. Materials that can drizzle or flow, such asquartz sand or glass balls, can be used as the electrically insulatingmaterial.

This has the advantage that the material that can drizzle or flow canpenetrate into the liquid metal after the electrically conducting piece,the solder, melts, and thus the formation of an arc light and thereby aflashover can be reliably prevented.

Advantageously, an encapsulation, for example of a heat-resistantplastic such as polyphenylene sulfide (PPS), can be provided thatcreates a container for the insulating material that can drizzle or flowand thereby simultaneously increase the mechanical stability of thecomponent.

The entire electroceramic component with the integrated temperature fuseand the encapsulation can be surrounded advantageously with a singlehousing. In this way, a compact component with low space requirement isformed.

In the following, the electroceramic component will be explained in moredetail with reference to the diagrams of embodiment examples.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a varistor with an integrated temperature fuse withtwo ceramic base bodies, in a top view and in cross-section.

FIGS. 2A and 2B show a varistor with only one ceramic base body, in atop view and in cross-section.

DETAILED DESCRIPTION

In FIGS. 1A and 1B, a series circuit with two varistor base bodies 1 and30, each with an operating voltage of about 60 to 75 V, is shown, sothat a total operating voltage of about 130 V can be achieved. The twovaristor base bodies 1 and 30 are connected to each other in anelectrically conducting manner by the electrically conducting piece 10,a low-melting solder with a melting point of about 80° C. to 180° C. Theelectrically insulating material 20 can be implemented as quartz sand,which is arranged between the two varistor base bodies and surrounds thesolder 10. Tinned copper wires, for example, can serve as electriccontacts 5 and 15. A plastic ring 50, made of a heat-resistant plasticsuch as polyphenylene sulfide (PPS), together with the two ceramic basebodies 1 and 30 as a lid, creates a hollow space for the insulatingmaterial 20. The hollow space can be closed by a stopper 50A. To preventexternal flashovers, the entire component with an integrated temperaturefuse can be surrounded advantageously by a housing 45 containing, forexample, epoxide plastic.

In case of a strong overvoltage, the electrically conducting piece 10 ofthe component melts reliably within a few seconds. At the time of thetriggering of the temperature fuse, the temperature at the housing ofthe component is only about 120° C. By this arrangement, it is assuredthat the component does not start to burn and also that no object in itsenvironment catches fire. At the same time, by using a solder wire about1 mm thick, current surges of some 8000 A (impulse form 8/20 μs) can bewithstood. This means that with the overheating fuse, no costs need tobe taken into account for a current bypass.

To modify the electric characteristics of the varistor, it is alsopossible to use two different varistor materials 1 and 30, for example,based on SiC.

The embodiment with the two ceramic base bodies also has the additionaladvantage that a spatial separation of the electrical contacts 5 and 15from the electrically conducting piece 10 is possible. The electricallyconducting piece is located in the intermediate space 35 between the twovaristor base bodies, whereas the electrical contacts contact the sides1A, 30A, facing away from the intermediate space in each case. Thereby,good thermal screening of the electrically conducting piece from theelectric contacts is provided so that a high resistance to heat from thesolder is provided. By this arrangement, in contrast to many traditionaltemperature fuses, problem-free soldering or welding of the electriccontacts is possible without triggering the temperature fuse.

FIG. 2A shows a top view of, and FIG. 2B a cross-section through, avaristor with only one varistor base body 1. In this case, theelectrically conducting piece 10 is in direct electric contact with thesecond electric contact 15 and the varistor base body 1. A ring 50, madeof plastic, creates a hollow space to receive the electricallyinsulating material 20, which, in this embodiment, is to prevent anelectric flashover between the varistor base body 1 and the secondelectric contact 15. The whole arrangement can be provided with a cover50B that closes the component. In addition, the varistor base body 1 iscontacted by the first electric contact 5.

This alternative embodiment shows similar characteristics in the case ofoverheating and similar current-diverting ability to that of theembodiment with two ceramic base bodies.

The electroceramic component is not limited to the examples that havebeen described herein. The electroceramic component may have manyadditional variations, especially in regard to the number of ceramicbase bodies used, their arrangement with respect to each other, and thetype of ceramic materials used.

1. An electric component comprising: a first base body, which iscomprised of ceramic; a first contact that is on, and that connects to,the first base body; a second base body, which is comprised of ceramic;a second contact that is on, and that connects to, the second base body;an electrically conducting element positioned along a current pathbetween the first contact and the second contact, the electricallyconducting element existing during normal operation of the electriccomponent, and the electrically conducting element being meltable due toheating of at least part of the electric component when an operatingvoltage of the electric component is exceeded; and an electricallyinsulating material substantially surrounding the electricallyconducting element and arranged so as to prevent an electric flashoverbetween regions of the current path that are bridged by the electricallyconducting element; wherein the electrically insulating material isbetween the first and second base bodies when the electricallyconducting element exists during normal operation.
 2. The electriccomponent of claim 1, wherein the electrically conducting element isthermally screened from the first and second contacts.
 3. The electriccomponent of claim 1, wherein: the first and second base bodies arebundled; the electrically conducting element is in a space between thefirst and second base bodies; and the first and second contacts contactthe first and second base bodies, respectively, on sides of the firstand second base bodies that face away from the space.
 4. The electriccomponent of claim 1, further comprising an encapsulation that createsand encloses a hollow space for the electrically insulating material. 5.The electric component of claim 4, wherein the encapsulation is heatresistant.
 6. The electric component of claim 4, further comprising ahousing that surrounds the first and second base bodies, theelectrically conducting element, the electrically insulating material,and the encapsulation, and that at least partially surrounds the firstand second contacts.
 7. The electric component of claim 1, wherein thefirst and second base bodies include a varistor ceramic.
 8. The electriccomponent of claim 7, wherein the varistor ceramic comprises ZnO.
 9. Theelectric component of claim 1, wherein the electrically insulatingmaterial drizzles or flows at least under certain circumstances.
 10. Theelectric component of claim 1, wherein the electrically insulatingmaterial comprises quartz sand or glass balls.
 11. The electriccomponent of claim 1, wherein the electrically conducting elementcomprises solder.
 12. The electric component of claim 1, wherein theelectrically conducting element comprises solder with a melting pointbetween about 80° C. and 180° C.